1. Field of the Invention
The present invention relates to an image developer suing a one-component developer, and more particularly to an image developer using a one-component developer developing an electrostatic latent image with a thin toner layer formed on a developing roller by pressing a toner thereon with a regulation blade.
2. Discussion of the Background
Conventional electrophotographic image forming methods include charging the surface of an image bearer (photoreceptor), irradiating the surface thereof to form an electrostatic latent image thereon, developing the electrostatic latent image with a colored toner to form a toner image thereon, transferring the toner image onto a receiving material such as a transfer paper, and fixing the toner image thereon.
Dry developing methods used in electrophotographic image forming methods and electrostatic recording methods include a method of using a two-component developer including a toner and a carrier, and a method of using a one-component developer not including a carrier. The former stably produces good images, but is difficult to produce constant-quality images for long periods because the carrier is easy to deteriorate and a mixing ratio of the toner to the carrier (toner concentration) is easy to vary. In addition, image forming apparatuses using the two-component developer are difficult to maintain and downsize. Therefore, the latter method of using the one-component developer is drawing attention.
The method of using the one-component developer typically feeds a toner (developer) with at least one toner transfer member and visualizes an electrostatic latent image formed on an image bearer with the toner. An amount of the toner on the toner transfer member is regulated by a toner layer thickness regulator located facing the toner transfer member, and the toner is charged when passing therethrough. Specific examples of thereof include various members such as a regulation blade. The regulation blade (a pressure member) is located facing the toner transfer member and presses the toner thereon to regulate the thickness of the toner layer. In addition, a roller instead of the blade is also widely used.
In the one-component developing method, the toner which has not been used in a developing area on the toner transfer member (a developing roller) is scraped by a toner feed member feeding a toner while contacting the toner transfer member, and is fed into a toner hopper again. However, when the adherence of the toner to the developing roller is large, the scrapability, i.e., the resettability deteriorates. Therefore, the toner on the developing roller increases, resulting in inability of stably charging and transferring the toner of the developing roller.
In order to solve this problem, various improvements are made to decrease the adherence between the developing roller and the toner.
Japanese published unexamined application No. 2006-99036 discloses an image developer regulating the resistivity of a developing roller to have good resettability. However, a relationship between the roller resistivity and an impedance of a toner is not regulated, charging and resetting are not well balanced, resulting in production of unstable-quality images.
Since a toner receives and leaves a charge contacting various members in image forming processes such as a stirring process, the measurement of a toner impedance against an AC voltage enables measurement of electrical properties of the toner moving the toner in an image forming apparatus. Particularly, in order to improve the resettability, the control of the impedance against an AC voltage is indispensable for production of stable-quality images because the electrical properties of a toner from the measurement of the toner impedance largely contribute thereto.
Japanese published unexamined application No. 2001-51495 discloses an image developer including a developing roller preferably having a resistivity of from 10×102 to 10×108. However, a relationship between the roller resistivity and an impedance of a toner is not regulated, charging and resetting are not well balanced, resulting in production of unstable-quality images as it is in the above-mentioned Japanese published unexamined application No. 2006-99036.
Further, Japanese published unexamined application No. 10-319709 discloses an image developer, wherein charging and resetting are not well balanced, either.
Furthermore, since adherence of an external additive to a parent toner has not conventionally regulated, a non-electrostatic adherence of a toner to a developing roller has not been controlled, resulting in occasional insufficient resettability of the toner. An external additive easily fixable on a parent toner and difficult to bury therein is necessary to control the non-electrostatic adherence of a toner so as to have stable resettability including durability.
Because of these reasons, a need exists for an image developer using one-component developer producing images not having uneven image density due to poor toner resettability and producing stable-quality images.